Optical device including zero-order imagery

ABSTRACT

An optical device including: a first surface; and an arrangement of pixels on the first surface, wherein a plurality of the pixels includes a zero-order diffraction element, such that each zero-order diffraction element is configured for providing a zero-order diffractive effect.

FIELD OF THE INVENTION

The invention generally relates to optical devices, in particularsecurity devices, for documents, such as banknotes.

BACKGROUND TO THE INVENTION

It is well known to include security features within documents requiringa level of security, for example banknotes. Such security features cantake on a number of forms, however particularly useful features are onesthat are visually apparent and, therefore, inspectable with relativeease.

However, unscrupulous counterfeiting groups have become better organisedand more technically competent, and the high returns fromcounterfeiting—in spite of the risks, have become more readilyappreciated. Over recent years, attempts at simulation of genuinedevices have become more and more successful. This problem isexacerbated by the fact that the authentication process for the banknoteby members of the public has long been recognised as the weakest pointin the security system. Often, such security features require inspectionby members of the public to be useful, but may be overly complicated tocorrectly view or may not provide a strong effect that is easilyrecognised. This diminishes the usefulness of such features in allowingthe public to take an active role in reducing the cost ofcounterfeiting.

Therefore, it is desirable to provide security features which aredifficult to reproduce and, therefore, counterfeit, while engaging thepublic such that regular authentication of banknotes can take place.Security features which provide a surprising visual effect, for examplerevealing a hidden image that is not normally visible, while notrequiring specialist equipment, are particularly desirable.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided anoptical device, preferably a security device for a security document,including: a first surface; and an arrangement of pixels on the firstsurface, wherein each pixel includes a zero-order diffraction element,such that each zero-order diffraction element is configured forproviding a zero-order diffractive effect, and wherein the arrangementof pixels is configured to provide an image, wherein the image includesan arrangement of microimages.

Preferably, the size of each pixel is the same. Each pixel may have adimension in the order of 5 to 100 microns.

In embodiments, each pixel has an associated brightness. The associatedbrightness of each pixel may be selected from one of a finite number ofbrightness levels, such as 16 brightness levels. Alternatively, theassociated brightness of each pixel may be selected from a continuousrange of brightness levels. The zero-order diffraction element of eachpixel may be located within an active region of the pixel, configuredsuch that the brightness of each pixel is determined by the size of theactive region of the pixel. The optical device may further include oneor more non-diffractive pixels, each non-diffractive pixel correspondingto a minimum brightness level.

Optionally, each zero-order diffraction element includes a periodicarrangement of grating elements. The period of the arrangement ofgrating elements for each zero-order diffraction element may be thesame. Preferably, the grating period is not greater than 500 nm, morepreferably not greater than 300 nm and even more preferably not greaterthan 250 nm. In embodiments, each zero-order diffraction element has acolour associated with it, and the period of the arrangement of gratingelements for each zero-order diffraction element is determined at leastin part based on the colour associated with it. The colour associatedwith each zero-order diffraction element may correspond to theappearance of the zero-order diffraction element when the optical deviceis viewed from a common position.

The grating elements of the optical device may have grating heights ordepths of 500 nm or less, preferably between 60 and 250 nm. In oneembodiment, the grating elements may have grating heights or depthsbetween 60 and 150 nm. Such a range of grating heights or depths can beused to generate special zero order colour effects depending on otherfactors such as grating period.

In an embodiment, the grating elements may have grating heights ordepths between 120 and 250 nm. The range of heights or depths can givevery bright diffraction efficiencies for high spatial frequencygratings, for example with grating periods of 250 nm or less.

The optical device optionally further includes a first opaque layer,optionally black or white, preferably white, applied to a second surfaceof the substrate opposite the first surface. In an alternative option,the optical device further includes an array of microlenses formed on asecond surface of the substrate, microlenses of the microlens arrayconfigured for viewing the arrangement of pixels. Where applicable, theoptical device may further include a second opaque layer, optionallyblack or white, preferably white, applied to the arrangement of pixelsthereby covering the arrangement of pixels.

According to a second aspect of the present invention, there is providedan optical system including an optical device according to the firstaspect and a verification device, the verification device including amicrolens array including an arrangement of microlenses, wherein themicrolens array is configured to provide an optical effect, preferably amoiré effect or an image switch effect, when positioned overlapping theoptical device such that the microlenses view the image

According to a third aspect of the present invention, there is provideda document, preferably a security document such as a banknote, includingthe optical device or optical system of the previous aspects.

According to a fourth aspect of the present invention, there is provideda method for manufacturing an optical device according to the firstaspect, the method including the steps of: applying a radiation curableink (RCI) to a first surface of a substrate; embossing the RCI using ahigh resolution embossing device; and curing the RCI.

The high resolution embossing device may be manufactured using a methodincorporating electron beam lithography. Electron beam lithography maybe utilised to create a master template, which is in turn may beutilised to manufacture the high resolution embossing device.

The method optionally includes a step of forming a microlens array,preferably an embossed microlens array, of a second surface of thesubstrate, such that microlenses of the microlens array are configuredfor viewing an image associated with the RCI.

According to fifth aspect of the present invention, there is provided amethod for manufacturing a document according to the third aspect,including the steps of: in a region of a substrate, applying a radiationcurable ink (RCI) to a first surface of a substrate, embossing the RCIusing a high resolution embossing device; and curing the RCI; andapplying to one or both of a first surface and a second surface of thesubstrate an opacifying layer, wherein the one or both opacifying layersare applied such that the RCI is visible from at least one side of thesubstrate.

Optionally, the method further includes the step of forming a microlensarray, preferably an embossed microlens array, in a different portion ofthe substrate to the RCI, such that when the banknote is folded orotherwise manipulated so that the microlens array is positionedoverlaying the RCI, microlenses of the microlens array are configuredfor viewing an image associated with the RCI. Alternatively, the methodmay further include a step of forming a microlens array, preferably anembossed microlens array, of a second surface of the substrateoverlapping the RCI, such that microlenses of the microlens array areconfigured for viewing an image associated with the RCI.

Security Document or Token

As used herein the term security documents and tokens includes all typesof documents and tokens of value and identification documents including,but not limited to the following: items of currency such as banknotesand coins, credit cards, cheques, passports, identity cards, securitiesand share certificates, driver's licenses, deeds of title, traveldocuments such as airline and train tickets, entrance cards and tickets,birth, death and marriage certificates, and academic transcripts.

The invention is particularly, but not exclusively, applicable tosecurity documents or tokens such as banknotes or identificationdocuments such as identity cards or passports formed from a substrate towhich one or more layers of printing are applied.

Substrate

As used herein, the term substrate refers to the base material fromwhich the security document or token is formed. The base material may bepaper or other fibrous material such as cellulose; a plastic orpolymeric material including but not limited to polypropylene (PP),polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC),polyethylene terephthalate (PET); or a composite material of two or morematerials, such as a laminate of paper and at least one plasticmaterial, or of two or more polymeric materials.

Transparent Windows and Half Windows

As used herein the term window refers to a transparent or translucentarea in the security document compared to the substantially opaqueregion to which printing is applied. The window may be fully transparentso that it allows the transmission of light substantially unaffected, orit may be partly transparent or translucent partially allowing thetransmission of light but without allowing objects to be seen clearlythrough the window area.

A window area may be formed in a polymeric security document which hasat least one layer of transparent polymeric material and one or moreopacifying layers applied to at least one side of a transparentpolymeric substrate, by omitting least one opacifying layer in theregion forming the window area. If opacifying layers are applied to bothsides of a transparent substrate a fully transparent window may beformed by omitting the opacifying layers on both sides of thetransparent substrate in the window area.

A partly transparent or translucent area, hereinafter referred to as a“half-window”, may be formed in a polymeric security document which hasopacifying layers on both sides by omitting the opacifying layers on oneside only of the security document in the window area so that the“half-window” is not fully transparent, but allows some light to passthrough without allowing objects to be viewed clearly through thehalf-window.

Alternatively, it is possible for the substrates to be formed from ansubstantially opaque material, such as paper or fibrous material, withan insert of transparent plastics material inserted into a cut-out, orrecess in the paper or fibrous substrate to form a transparent window ora translucent half-window area.

Opacifying Layers

One or more opacifying layers may be applied to a transparent substrateto increase the opacity of the security document. An opacifying layer issuch that L_(T)<L₀, where L₀ is the amount of light incident on thedocument, and L_(T) is the amount of light transmitted through thedocument. An opacifying layer may comprise any one or more of a varietyof opacifying coatings. For example, the opacifying coatings maycomprise a pigment, such as titanium dioxide, dispersed within a binderor carrier of heat-activated cross-linkable polymeric material.Alternatively, a substrate of transparent plastic material could besandwiched between opacifying layers of paper or other partially orsubstantially opaque material to which indicia may be subsequentlyprinted or otherwise applied.

Security Device or Feature

As used herein the term security device or feature includes any one of alarge number of security devices, elements or features intended toprotect the security document or token from counterfeiting, copying,alteration or tampering. Security devices or features may be provided inor on the substrate of the security document or in or on one or morelayers applied to the base substrate, and may take a wide variety offorms, such as security threads embedded in layers of the securitydocument; security inks such as fluorescent, luminescent andphosphorescent inks, metallic inks, iridescent inks, photochromic,thermochromic, hydrochromic or piezochromic inks; printed and embossedfeatures, including relief structures; interference layers; liquidcrystal devices; lenses and lenticular structures; optically variabledevices (OVDs) such as diffractive devices including diffractiongratings, holograms and diffractive optical elements (DOEs).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings. It is to be appreciated that the embodiments aregiven by way of illustration only and the invention is not limited bythis illustration. In the drawings:

FIGS. 1a to 1c each show a document including an optical device;

FIG. 2 shows an optical device according to an embodiment;

FIGS. 3a and 3b show pixels according to different embodiments;

FIG. 4 shows an arrangement of grating elements of a zero-order pixel;

FIG. 5 shows pixels arranged into groups comprising pixels of differentcolours; and

FIGS. 6a to 6b show embodiments incorporating arrangements ofmicrolenses.

DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIGS. 1a and 1 b, there is provided a document 2including an optical device 4, such as a security device, and anoptional verification feature 6. The document 2 can be a securitydocument 2, such as a banknote. The security document 2 can also be anyother document which requires a level of security, for example a creditcard or passport. The document 2 includes a substrate 8, which caninclude a first opacifying layer 10 applied to a first side 11 and asecond opacifying layer 12 applied to a second side 13. Both the firstopacifying layer 10 and the second opacifying layer 12 are shownincluding window regions corresponding to the optical device 4 and theverification device 6, however it is noted that in some configurationsone of the first and second opacifying layers 10, 12 can be configuredto cover one of the optical device 4 and the verification device 6, suchas shown in FIG. 1c where the second opacifying layer 12 is showncovering the optical device 4. In the case of FIG. 1 c, the opacifyinglayer can correspond to a opaque backing for the optical element 4, suchas a white or black backing. In this way, the optical device 4 or theverification device 6 can be formed in a half-window region.

With reference to FIG. 2, the optical device 4 includes a substrate 8having a first surface 16 a and a second surface 16 b, corresponding tothe first and second sides 11, 13 of the document 2, respectively. Thefirst surface 16 a includes an arrangement of pixels 14. As shown in thefigure and assumed herein, the arrangement of pixels 14 corresponds to aregular 2D array of pixels 14, however in general the arrangement ofpixels 14 can be any suitable arrangement, including a non-regulararrangement. The pixels 14 are arranged in order to form an image whichis viewable by a user, or a hidden image which must be revealed by useof one or more verification devices 6. It is understood that the imageor hidden image may correspond to an arrangement of microimages, such asa repeating 1D or 2D pattern of microimages. The pixels 14 can each bethe same size, wherein the ‘size’ of a pixel 14 as used hereincorresponds to the area that the pixel 14 takes up on the first surface16 a.

Referring to FIGS. 3a and 3b , each pixel 14 includes a zero-orderdiffraction element 18 (in the figures the zero-order diffractionelement 18 constitutes the shaded portion of the pixel 14), configuredfor providing a zero-order diffraction visual effect. In the embodimentshown in FIG. 3a , the zero-order diffraction element 18 corresponds tothe entire pixel 14. In the embodiment shown in FIG. 3b , each pixel 14has an active region 20, wherein the diffractive element 18 is locatedwithin the active region 20. The portion of each pixel 14 not includingthe active region 20 is herein labelled the inactive region 22 of thepixel. As shown in FIG. 3b , different pixels 14 a, 14 b, 14 c can havedifferently sized active regions 20 a, 20 b, 20 c. Different sizes ofactive regions 20 result in different brightness of the correspondingpixels 14, with a large active region 20 associated with a brighterpixel 14. ‘Brightness’ as used herein corresponds to relative brightnessbetween pixels 14. Preferably, maximum brightness corresponds to thelargest active region 20 associated with a pixel 14. Also shown in FIG.3b is a non-diffractive pixel 15. The non-diffractive pixel 15corresponds to a pixel 14 with only an inactive region 22. Thenon-diffractive pixel 14 therefore corresponds to a minimum brightnesspixel 14. Each pixel 14 can have a brightness selected from a finiterange of brightness levels (e.g. 16 levels), or alternatively, thebrightness of each pixel 14 is selected from a continuous range ofbrightness levels.

Referring to FIG. 4, each zero-order diffraction element 18 includes anarrangement of grating elements 24. In the configuration shown, thegrating elements 24 correspond to projections from the first surface 16a of the optical device 4. Other configurations include grating elements24 corresponding to grooves or depressions in the first surface 16 a, orareas of different refractive index when compared to the substrate 8 inwhich the grating elements 24 are embedded, or a layer applied to thesubstrate 8 in which the grating elements 24 are embedded. As shown, thegrating elements 24 are present in a linear periodic arrangement with aconstant grating element period 26 and a constant grating element heightor depth. For example, the grating period is below 500 nm, preferablybelow 300 nm, and more preferably below 250 nm. Grating heights ordepths are, preferably, 500 nm or less, and more preferably between 60and 250 nm. In some embodiments, the grating heights or depths may bebetween 60 and 150 nm, or between 120 nm and 250 nm, depending on thezero-order effects required. The pitch and widths of the gratingelements is preferably 500 nm or less, and more preferably between 60and 250 nm.

In an embodiment, each zero-order diffraction element 18 of the opticaldevice 4 has a common constant grating element period 26, and a commongrating alignment. An image is provided due to variation in thebrightness of each pixel 14 based on the size of an active region 20 asdescribed with reference to FIG. 3b . For example, for a monochromatic2-colour image, each pixel 14 can be selected to have one of twobrightness levels. In a particular implementation of this example, onebrightness level corresponds to a pixel 14 with no inactive region 22and the other brightness level corresponds to a pixel 14 with no activeregion 20 (i.e. a non-diffractive pixel 15). In another example, a16-colour image can be created where each pixel 14 has a brightnesslevel selected from one of 16 levels (where the minimum brightness levelcan correspond to a non-diffractive pixel 15). In this embodiment, whenthe optical device 4 is viewed from a predetermined position, theoptical device 4 may appear to as a monochromatic colour image. Thecolour of the image is at least determined by the common grating period26, and may further be determined by choice of: substrate 8 material,grating element 24 material, coating between the substrate 8 and gratingelements 24, coating covering the grating elements 24, etc. In general,for a particular optical device 4, the colour can be determined throughroutine experimental variation of grating period 26.

Another embodiment corresponds to a variation of the previouslydescribed embodiment. In this embodiment, each pixel 14 can have acolour selected from two or more colours. The colour of each pixel 14corresponds to the colour of the pixel 14 when viewed from apredetermined common viewing position. In one implementation of thisembodiment, each pixel 14 has a colour selected from one of threecolours, namely red, green, and blue. Each pixel 14 further has anassociated brightness as previously described. In this way, an RGB imagecan be produced. As shown in FIG. 5, the pixels 14 are arranged intogroups 28 including pixels 14 associated each possible colour (red,green, blue). In order to maintain a regular 2D arrangement of pixels14, there may be two of pixels of a colour in a group 28 (such as thetwo green pixels shown in FIG. 5).

Referring to FIGS. 6a to 6c , a microlens array 30 is provided forviewing the pixels 14 of a pixel layer 30. In FIG. 6a , the microlensarray 30 is provided on the opposite surface (second surface 16 b) ofthe substrate 8 to the pixel layer 30, and configured for focussing onthe pixels 14 of the pixel layer 30. In FIG. 6b , the microlens array 30is provided in a separate portion of the substrate 8 to the microlensarray 30, thereby forming a verification element of a verificationdevice 32.

In FIG. 6c , the microlens array 30 is provided as a verification devicecorresponding to the verification feature 6 of the document 2. In thiscase, the microlenses of the microlens array 30 are configured forfocussing on the pixels 14 of the optical device 4 when the document 2is folded or otherwise manipulated such that the microlens array 30 isoverlapping the optical element 4, preferably in contact with either thefirst side 11 or second side 13.

The microlens array 30 is suitable for viewing an arrangement an imagecorresponding to an arrangement of microimages. An advantage of pixels14 having zero-order diffraction elements 18 is that high resolutionimagery is possible. Zero-order diffraction elements 18 are advantageousin comparison to first and higher order diffraction elements as it hasbeen found that microlenses act to recombine first and higher orderdiffraction effects, thereby reducing the effectiveness of such gratingsfor use in microlens and microimage based optical devices. Therefore,zero-order diffraction ratings 18 can provide for high contrast, highresolution microimagery. High resolution imagery can correspond topixels with a dimension in the order of 5 to 100 microns. For example, asquare pixel can have a length and breadth each of 5 to 100 microns. Acircular pixel can have a diameter of 5 to 100 microns. Decreasing pixelsize affects the amount of light that each individual pixel reflectsand, therefore, the particular application will determine the ideal sizeof the pixel.

As the grating spacing of the zero-order grating elements 18 of thepixels 14 is relatively low, high resolution techniques are required forforming the pixels 14. One such technique for forming the pixels 14 usesembossing with a high resolution embossing device. The high resolutionembossing device can be created with a method incorporating electronbeam lithography, which enables the formation of high detail (andtherefore high resolution) features on a surface. A master template canbe created using electron beam lithography, which can then be utilisedto create the high resolution embossing device. The arrangement ofpixels 14 can be formed by first applying a radiation curable ink (RCI)to a first surface of the substrate 8, and embossing the radiationcurable ink using the embossing tool. Due to surface tension effects, itmay be desirable to cure the RCI before removing the embossing tool,such that the structure of the zero-order grating elements 18 ismaintained. The RCI is preferably cured using appropriate radiation, forexample a UV curable ink can be cured by exposure to UV radiation. It isunderstood that other inks and curing methods can be used, for exampleheat curable inks.

Further modifications and improvements may be made without departingfrom the scope of the present invention.

1.-25. (canceled)
 26. An optical device including: a first surface; andan arrangement of pixels on the first surface, wherein a plurality ofthe pixels includes a zero-order diffraction element, such that eachzero-order diffraction element is configured for providing a zero-orderdiffractive effect, and wherein the arrangement of pixels is configuredto provide an image, wherein the image includes an arrangement ofmicroimages.
 27. An optical device as claimed in claim 26, wherein thesize of each pixel is the same and each pixel has a dimension of 5 to500 microns.
 28. An optical device as claimed in claim 26, wherein eachpixel has an associated brightness, the associated brightness of eachpixel being selected from one of a finite number of brightness levelsand/or from a continuous range of brightness levels.
 29. An opticaldevice as claimed in claim 28, wherein the zero-order diffractionelement of each pixel is located within an active region of the pixel,configured such that the brightness of each pixel is determined by thesize of the active region of the pixel.
 30. An optical device as claimedin claim 28, further including one or more non-diffractive pixels, eachnon-diffractive pixel corresponding to a minimum brightness level. 31.An optical device as claimed in claim 26, wherein each zero-orderdiffraction element includes a periodic arrangement of grating elementsand the period of the arrangement of grating elements for eachzero-order diffraction element is the same.
 32. An optical device asclaimed in claim 31, wherein each zero-order diffraction element has acolour associated with it, and wherein the period of the arrangement ofgrating elements for each zero-order diffraction element is determinedat least in part based on the colour associated with it.
 33. An opticaldevice as claimed in claim 32, wherein the colour associated with eachzero-order diffraction element corresponds to the appearance of thezero-order diffraction element when the optical device is viewed from acommon position.
 34. An optical device as claimed in claim 31, whereinthe grating elements have grating depths or heights of 500 nm or less,preferably between 60 and 250 nm.
 35. An optical device as claimed inclaim 26, further including an array of microlenses formed on a secondsurface of the substrate, wherein the first and second surfacescorrespond to opposite sides of a transparent or translucent substrate,wherein the array of microlenses are configured for viewing thearrangement of pixels.
 36. An optical device as claimed in claim 26,further including a first opaque layer, optionally black or white,preferably white, applied to the arrangement of pixels thereby coveringthe arrangement of pixels.
 37. An optical system including an opticaldevice as claimed in claim 26 and a verification device, theverification device including a microlens array including an arrangementof microlenses, wherein the microlens array is configured to provide anoptical effect, preferably a moiré effect or an image switch effect,when positioned overlapping the optical device.
 38. A document,preferably a security document such as a banknote, including the opticaldevice as claimed in claim
 26. 39. A method for manufacturing an opticaldevice as claimed in claim 26, the method including the steps of:applying a radiation curable ink (RCI) to a first surface of asubstrate; embossing the RCI using a high resolution embossing device;and curing the RCI.
 40. A method as claimed in claim 39, wherein thehigh resolution embossing device is manufactured using a methodincorporating electron beam lithography.
 41. A method as claimed inclaim 40, wherein electron beam lithography is utilised to create amaster template, which is in turn utilised to manufacture the highresolution embossing device.
 42. A method as claimed in claim 41,including a step of forming a microlens array, preferably an embossedmicrolens array, of a second surface of the substrate, such thatmicrolenses of the microlens array are configured for viewing an imageassociated with the RCI.
 43. A method for manufacturing a document asclaimed in claim 40, including the steps of: in a region of a substrate,applying a radiation curable ink (RCI) to a first surface of asubstrate, embossing the RCI using a high resolution embossing device;and curing the RCI; and applying to one or both of a first surface and asecond surface of the substrate an opacifying layer, wherein the one orboth opacifying layers are applied such that the RCI is visible from atleast one side of the substrate.
 44. A method as claimed in claim 43,further including the step of forming a microlens array, preferably anembossed microlens array, in a different portion of the substrate to theRCI, such that when the banknote is folded or otherwise manipulated sothat the microlens array is positioned overlaying the RCI, microlensesof the microlens array are configured for viewing an image associatedwith the RCI.
 45. A method as claimed in claim 43, including a step offorming a microlens array, preferably an embossed microlens array, of asecond surface of the substrate overlapping the RCI, such thatmicrolenses of the microlens array are configured for viewing an imageassociated with the RCI.